1. Field of the Invention
This invention relates to pneumatic tires. In particular it relates to an improvement in shape of a main circumferential groove in a pneumatic tire for use in vehicles such as trucks, buses, and the like, that run on both paved roads and gravel roads.
2. Description of the Prior Art
Generally, a main groove in a tread of a pneumatic tire for use in vehicles such as trucks, buses, and the like, is made deep to prolong the wear life of the tire. The width of the main groove is restricted to be less than a predetermined limit to provide sufficient rubber to be worn during the life of the tire. Therefore, there is a problem in the sectional shape of the main groove since the groove depth is deep and the groove angle is small for the groove width. Such a groove is apt to seize and retain stones. The larger the ratio of the groove width W to the depth H (W/H), the possibility of stone seizing by the groove is decreased. However, if the groove depth is made deeper with ratio W/H kept constant, the groove width becomes wide, so that the amount of rubber to be worn out is decreased thus reducing the lifetime of the tire. Accordingly, the ratio W/H of groove width to the depth is in prior art tires generally selected to be about 0.5 -0.8.
Conventionally, as illustrated in FIGS. 2 and 3 for example, a pneumatic tire 6 has a main groove 1 having a substantially U-shaped section and circumferentially formed in an outer surface region 2a of a tread 2. Each of the opposite side wall surfaces 3 of the main groove 1 is slanted to form an angle .alpha..sub.1 of, for example, 13 degrees with a normal line 5 perpendicular to an outer surface 2b of the tread 2.
However, when the pneumatic tire 6 having such a main groove 1 is driven on a road 7 where pieces of stone 9 are scattered, the opening of the groove 1 is widened at a side 8a of the tire 6 (FIG.. 3) to seize the stone 9. The stone 9 once seized in the main groove 1 is pushed further into the main groove 1 at a portion just below portion of a load 8b. It is thus urged into the main groove 1 between the wall surfaces 3 thereof, so that even after the main groove 1 rotates away from a road surface 7, a large retaining force F.sub.1 is exerted onto the stone 9 at the outer surface 2b of the tread 2. This is illustrated in FIG. 4. The force F.sub.1 prevents the stone 9 from being discharged from the main groove 1.
FIG. 5, illustrated another pneumatic tire provided with a main groove 11 in which each of the opposite side walls of the groove 11 are inclined at compound angles in an opening portion 11a of the groove 11. Two side wall angles .alpha..sub.1 and .alpha..sub.2 formed between a side groove wall surface 12 at the opening portion 11a and a normal line 5. The angle .alpha..sub.2 is about 25 degrees. In such a main groove 11 having such an opening portion 11a, the groove 11 does not readily seize a stone because of the enlarged opening of the groove. However, when a stone is pushed into an inner portion of the groove from the opening, the stone is pushed farther into the inner portion of the main groove 11. The stone cannot be discharged from the groove because a fastening force is exerted onto the stone from the wall surfaces as in the case of the main groove 1 of the conventional tire as described with reference to FIGS. 2-4. There is therefore a problem that if the groove is formed with the groove wall angle at the opening portion 11a as illustrated, the groove depth becomes so shallow that the life of the tire is reduced. That is, there is an inverse relationship between the groove depth and the stone-seizing characteristic of the groove. Also, as the tire wears, when the tread gauge 22 is reduced, the groove depth decreased eliminating walls 12 and angle .alpha..sub.2. The groove 3 is then identical to FIG. 2.